Air Stable N-doping of Wse 2 by Silicon Nitride Thin Films with Tunable Fixed Charge Density

Stable n-doping of WSe 2 using thin films of SiN x deposited on the surface via plasma-enhanced chemical vapor deposition is presented. Positive fixed charge centers inside SiN x act to dope WSe 2 thin flakes n-type via field-induced effect. The electron concentration in WSe 2 can be well controlled up to the degenerate limit by simply adjusting the stoichiometry of the SiN x through deposition process parameters. For the high doping limit, the Schottky barrier width at the metal/WSe 2 junction is significantly thinned, allowing for efficient electron injection via tunneling. Using this doping scheme, we demonstrate air-stable WSe 2 n-MOSFETs with a mobility of ∼70 cm 2 /V s. © 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. Two dimensional (2D) layered materials have generated much interest due to the ability to obtain atomically well-defined layers with thicknesses down to a single layer, providing a potential pathway to ultra-scaled transistors with excellent gate control. 1–4 In addition, they may enable new electronic and optoelectronic device architectures with unique functionalities. 5, 6 Transition metal dichalcogenides (TMDCs) as a class of 2D materials, in particular, exhibit a wide range of electronic properties ranging from metallic to insulating, 7 with many being semiconductors with intrinsic carrier mobilities >100 cm 2 /V s 8 even when scaled down to a monolayer in thickness. It should be noted that the measured mobility is high for a sub-1 nm thick semiconductor when compared to silicon or III–V semiconductors of similar thicknesses, 9 owing to the lack of surface native oxides and in principle atomically smooth surfaces which reduce surface roughness scattering of carriers. Due to this range of available materials, transistors have been made consisting of entirely 2D materials, exhibiting proof of concept for the scaling potential of layered materials. 10 However, one challenge of TMDCs is that they generally exhibit a high degree of Fermi level pinning, 11 resulting in it being very difficult to contact both the conduction band and valence band for the same material. In this regard, degenerate doping of the source/drain contacts, similar to what is often used in conventional 3D semiconductors, is the most effective route for injecting electrons or holes with low contact resistances. Furthermore, controlled n-and p-doping of TMDCs is critical for enabling various device structures. Doping of TMDCs via surface charge transfer doping has …

2][3][4] In addition, they may enable new electronic and optoelectronic device architectures with unique functionalities. 5,6 ransition metal dichalcogenides (TMDCs) as a class of 2D materials, in particular, exhibit a wide range of electronic properties ranging from metallic to insulating, 7 with many being semiconductors with intrinsic carrier mobilities >100 cm 2 /V s 8 even when scaled down to a monolayer in thickness.It should be noted that the measured mobility is high for a sub-1 nm thick semiconductor when compared to silicon or III-V semiconductors of similar thicknesses, 9 owing to the lack of surface native oxides and in principle atomically smooth surfaces which reduce surface roughness scattering of carriers.Due to this range of available materials, transistors have been made consisting of entirely 2D materials, exhibiting proof of concept for the scaling potential of layered materials. 10However, one challenge of TMDCs is that they generally exhibit a high degree of Fermi level pinning, 11 resulting in it being very difficult to contact both the conduction band and valence band for the same material.In this regard, degenerate doping of the source/drain contacts, similar to what is often used in conventional 3D semiconductors, is the most effective route for injecting electrons or holes with low contact resistances.Furthermore, controlled n-and p-doping of TMDCs is critical for enabling various device structures.Doping of TMDCs via surface charge transfer doping has been demonstrated in the past using a variety of electron donating or withdrawing molecular species. 8,12,13 Wile the previous works have demonstrated the proof-of-concept viability of n-and p-doping for TMDCs, development of air-stable and scalable dopant processes is still of significant need.
In this work, we present a robust n-doping method for WSe 2 by depositing thin films of silicon nitride on the surface using plasma enhanced chemical vapor deposition (PECVD).PECVD nitride has been widely used for field-effect passivation of silicon solar cells [14][15][16] and is an air stable 14,17 and scalable process. 18Silicon nitride, as grown by PECVD, contains a high density of positive charge centers which originate from + Si≡N 3 dangling bonds known as K + centers, 19 shown in Figure 1(a), with their density being controllable by the deposition process parameters.This characteristic is utilized in Si solar cells which induces a favorable surface band bending of Si, and thus reduces the surface recombination velocity of carriers.1][22] Here, we apply the same concept to WSe 2 devices.We observe strong field-induced electron doping of WSe 2 by SiN x coating (Figure 1(b)), with the electron sheet density being user-tunable, up to the degenerate limit by controlling the stoichiometry of the PECVD SiN x .Notably, given the small thickness of WSe 2 (from monolayer up to ∼10 nm explored here), the entire body of WSe 2 can be inverted to n-type by utilizing this surface-induced field-effect doping scheme.This has enabled us to fabricate stable n-type WSe 2 transistors with excellent performance.More generally, the approach here presents a route towards controlling the electronic properties of TMDCs by simply depositing thin films of charged dielectrics on the surface.Deposition of the SiN x was done using an Oxford Plasmalab 80plus PECVD system at 13.56 MHz frequency.The deposition power, temperature, pressure, and time were kept constant for all processes at 20 W, 150 • C, 900 mTorr, and 2 min, respectively, which results in a SiN x thickness of ∼50 nm.Silane (SiH 4 , 10% in Ar) and ammonia (NH 3 ) were used as the process gasses and the concentrations were varied to obtain different film stoichiometries and amount of fixed charge.In this paper, the NH 3 /SiH 4 ratio refers to the ratio of NH 3 to the 10% SiH 4 in Ar gas mixture.The four different NH 3 /SiH 4 gas flow ratios explored were 1/10, 1/2, 1/1, and 3.3/1, corresponding to absolute NH 3 /SiH 4 flow rates of 30/300, 50/100, 100/100, and 100/30 sccm, respectively.
To test the effect of different ratios of NH 3 to SiH 4 on the doping of WSe 2 , back-gated WSe 2 transistors were fabricated with the device structure shown in Figure 2(a).First, WSe 2 (HQ Graphene) was mechanically exfoliated onto a heavily doped p-type silicon wafer (resistivity 0.001-0.005cm) with 260 nm of thermal silicon oxide, which acts as a universal back gate.To ensure that the thickness of the WSe 2 did not contribute significantly to device to device variation, the WSe 2 flakes used for devices were kept approximately constant between 5 and 10 nm in thickness via optical contrast. 23After exfoliation, Au source (S) and drain (D) contacts were patterned and deposited using photolithography and electron-beam evaporation, respectively.Finally, SiN x (∼50 nm in thickness) was deposited via PECVD on top of the entire sample to dope the devices.
The width normalized transfer characteristics of the WSe 2 devices at V DS = −1 V after nitride deposition are shown in Figure 2(b) for multiple NH 3 /SiH 4 process concentrations corresponding to different SiN x stoichiometries.All devices were fabricated with a channel length of 2 μm.A representative I DS -V GS curve for a device before SiN x deposition is also shown.As expected, the asmade device exhibits ambipolar transfer characteristics with slightly higher p-channel conductance, corresponding to close to mid-gap Schottky barriers at the source/drain metal contacts.This behavior is similar to other previous reports on WSe 2 devices with Au metal contacts. 8Upon deposition of SiN x , the devices exhibit n-type transfer characteristics with a shift in the threshold voltage towards a more negative voltage.This result is indicative of electron doping of WSe 2 by SiN x .The electron doping effect depends on the NH 3 /SiH 4 ratio used during SiN x deposition.As the NH 3 /SiH 4 concentration ratio was increased, the devices become more heavily n-doped with higher n-channel conductance.Specifically, for NH 3 /SiH 4 ratios of >1/1, WSe 2 becomes degenerately doped with minimal gate dependence in the transfer curves (Fig. 2(b)).The two dimensional electron sheet densities (n 2D ) of the devices can be estimated from n 2D = (I DS L)/(qμWV DS ), 13 where I DS is the drain current at zero gate voltage, L and W are the lengths and widths of the devices, respectively, q is the electron charge, V DS is the applied drain voltage, and μ is the field-effect mobility.It should be noted that this calculation does not take into account the effect of contact resistance, which may underestimate n 2D , especially for the lower doped devices.From Figure 2(c), it can be seen that the sheet density varies from 7.4 × 10 11 to 9.5 × 10 13 cm −1 for the explored NH 3 /SiH 4 ratio.The ability to tune the electron concentration in WSe 2 by orders of magnitude by simply changing the stoichiometry of SiN x presents a facile doping scheme.enables greater electron injection by tunneling through the barrier and hence forms a practical ohmic contact for electrons (Fig. 2(d)).
Figure 3(a) shows the transfer characteristics at V DS = −0.05 and −1 V of a WSe 2 back-gated transistor two weeks after deposition of SiN x with a NH 3 /SiH 4 ratio of 1/2, which corresponds to a WSe 2 sheet electron density of 2 × 10 12 cm −2 (Fig. 2(c)).This electron sheet density is high enough to provide a low resistance metal contact to the conduction band of WSe 2 while still allowing for gate control of the channel.The transfer characteristics of the device before doping are included for reference.An optical microscope image of the actual device is shown in the inset.This n-type FET exhibits a peak field-effect mobility of ∼70 cm 2 /V s as extracted using the analytical equation μ = G m L/(WV DS C ox ).Here, G m is the transconductance, C ox = 1.33 × 10 −8 F/cm 2 is the calculated gate oxide capacitance for 260 nm of SiO 2 , and W and L are the width and length of the channel, respectively.Figure 3(b) shows the output characteristics of the same device after nitride deposition.As can be seen, the curves are linear at low V DS , indicating that the contact is near ohmic.
To understand the effect of the NH 3 /SiH 4 concentration during deposition on the doping level, the fixed charge density present inside the nitride was extracted using capacitance-voltage (C-V) measurements.A metal insulator semiconductor (MIS) structure with an Al gate on top of SiN x , shown in the inset of Figure 4(a), was fabricated for the C-V measurements.First, the SiN x was deposited via PECVD onto a lightly doped n-type silicon wafer with resistivity between 10 and 30 cm.Then, 500 × 500 μm squares of Al (150 nm in thickness) were deposited onto the nitride using a shadow mask via thermal evaporation.As can be seen in Figure 4(a), the flat band voltages of the MIS structures shift more negatively as the NH 3 /SiH 4 ratio is increased for deposition.In all cases, the flat band voltage is negative with respect to the expected flat band voltage of Al and the lightly doped n-type wafer, given by the work function difference of φ MS ∼ −0.23 V.This indicates the presence of positive fixed charge inside the nitride as expected.The flat band voltage, V FB , can be determined as the peak of d 2 (1/C nitride 2 )/dV 2 where C nitride is the accumulation capacitance of the MIS structure. 24For 1/10, 1/2, 1/1, and 3.3/1 NH 3 /SiH 4 ratios, the measured V FB are approximately −4.3, −6.2, −7.4, and −9.1 V, respectively.The fixed charge density can be calculated from the shift in the V FB using the equation Q F = C nitride (φ MS − V FB )/q assuming that the fixed charge is equally distributed throughout the nitride where q is the electron charge.The calculated fixed charge densities vs. the NH 3 /SiH 4 gas flow ratio used during deposition is plotted in Figure 4(b).As can be seen, the fixed charge densities increase monotonically as the ratio of NH 3 to SiH 4 is increased, which results in the higher doping levels observed in these devices.
Fourier transform infrared spectroscopy (FTIR) measurements were conducted on SiN x films deposited at different NH 3 /SiH 4 concentration ratios to measure the relative amounts of Si-H bonds.mode. 25As can be seen, the integrated Si-H peak intensity drops as the NH 3 /SiH 4 ratio is increased, indicating a reduced amount of Si-H bonds.In addition, the peak position shifts from ∼2129 cm −1 at 1/10 NH 3 /SiH 4 to ∼2190 cm −1 at 3.3/1 NH 3 /SiH 4 which corresponds to an increase in the N 3 Si-H bonding configuration component of the Si-H stretching mode 26 which has a peak position at 2220 cm −1 .This peak shift also indicates a stoichiometry change from silicon rich to a more nitrogen rich SiN x . 27The drop in integrated peak intensity and shift towards higher wavenumbers has been correlated with an increased density of dangling bond K centers. 27ue to the use of a plasma during the nitride deposition, it is possible that the surface of the WSe 2 can be damaged.To explore the effect of the PECVD deposition on the integrity of WSe 2 , a backgated monolayer WSe 2 transistor with Pd S/D contacts was fabricated.As the device is a single layer, any surface damage would result in drastic changes to its electronic and optoelectronic properties.An optical image of the device after doping is shown in Figure 5(a).The transfer characteristics of the monolayer before and after nitride deposition using an NH 3 /SiH 4 ratio of 1/2 are plotted in Figure 5(b).Just as with the thicker WSe 2 devices, the monolayer device shows good n-type behavior after nitride deposition.To further explore the effect of the PECVD on the WSe 2 , photoluminescence (PL) and Raman spectroscopy (HORIBA LabRAM HR800, 532 excitation wavelength) were carried out before and after nitride deposition.To ensure that no damage or measurement artifacts would result from sample heating, the laser power used was 0.8 mW.As seen in Figure 5(c), after SiN x deposition, a redshift of 16 meV is observed in the PL peak energy in addition to a 35× decrease in the peak intensity.9][30] Similarly, in Figure 5(d), a 5× decrease in the Raman intensity as well as a peak shift of 0.7 cm −1 of the E 1 2g raman peak can be attributed to a softening of the vibrational modes due to the increase in electron concentration, which has been observed in monolayer MoS 2 devices. 12,31 verall, no unexpected change in the electronic and optoelectronic properties of the monolayer WSe 2 was observed after SiN x deposition, indicating that the deposition process does not affect the crystalline integrity of the WSe 2 .
In conclusion, we have demonstrated an air stable n-doping scheme for TMDCs using PECVD SiN x with tunable fixed charge density arising from positively charged + Si≡N 3 dangling bonds.By tuning the gas flow ratios of NH 3 and SiH 4 , different densities of fixed charge within the nitride up to 1.75 × 10 12 cm −2 are demonstrated, allowing for a controlled method to change the doping concentration via field-effect.Using this doping scheme, WSe 2 n-MOSFETs were fabricated, exhibiting a high electron mobility of ∼70 cm 2 /V s.Similar to that of modulation doping utilized in high electron mobility transistors where the dopant atoms are away from the actual channel material, field-effect doping can enable high doping concentrations in TMDCs without causing damage to the lattice nor experiencing a drop in mobility due to ionized impurity scattering.Given the ease of processing of SiN x and its stability as an inorganic thin film, the approach here presents a robust

FIG. 1 .
FIG. 1.(a) Diagram of the positively charged K + center found inside silicon nitride, originating from Si + ≡N 3 dangling bonds.(b) Schematic of the doping mechanism of the SiN x .The positively charged K + centers act as fixed charges which attract electrons inside the WSe 2 , thus inverting the material.

FIG. 2 .
FIG. 2. (a) Schematic of the back gated WSe 2 device structure used to test the effect of NH 3 /SiH 4 ratios during nitride deposition on doping.(b) Width normalized transfer characteristics of four back gated WSe 2 devices with nitride deposited using different NH 3 /SiH 4 ratios.As the NH 3 /SiH 4 ratio is increased, the doping level increases.To minimize the effect of device to device variation, the WSe 2 thickness was kept within the range of 5-10 nm.A representative transfer characteristic curve of a device before nitride deposition is also shown for reference.(c) Extracted two dimensional electron sheet charge densities for the four nitride deposition conditions at zero gate bias.(d) Qualitative band diagrams of the back gated devices before and after SiN x deposition.After SiN x deposition, the Schottky barrier width at the metal/WSe 2 contact decreases, thus enabling efficient injection of electrons by tunneling.
Figure 2(d) depicts a qualitative band diagram of the effect of SiN x on WSe 2 .As the devices become increasingly doped by the fixed charges in SiN x , the Fermi level moves closer to the conduction band, shifting the threshold voltage negatively as well as reducing the width of the metal/WS 2 Schottky barrier.The thinner barrier This article is copyrighted as indicated in the article.Reuse of AIP content is subject to the terms at: http://aplmaterials.aip.org/about/rights_and_permissionsDownloaded to IP: 128.32.32.134On: Tue, 05 Aug 2014 15:57:07

FIG. 3 .
FIG. 3. (a) Transfer characteristics of a WSe 2 back-gate FET before and after SiN x deposition at a NH 3 /SiH 4 concentration ratio of 1/2.An optical image of the device is shown in the inset.(b) Output characteristics of the device post SiN x deposition.

Figure 4 (
c) shows the FTIR spectra at ∼2100 cm −1 , which corresponds to the Si-H stretching This article is copyrighted as indicated in the article.Reuse of AIP content is subject to the terms at: http://aplmaterials.aip.org/about/rights_and_permissionsDownloaded to IP: 128.32.32.134On: Tue, 05 Aug 2014 15:57:07

FIG. 4 .
FIG. 4. (a) Capacitance-voltage (C-V) curves for silicon nitride films deposited at different NH 3 /SiH 4 concentration ratios.The device schematic is shown in the inset.(b) Plot of the extracted fixed charge density in the silicon nitride films as a function of NH 3 /SiH 4 ratio.As the ratio is increased, the fixed charge density increases.(c) Fourier transform infrared spectroscopy of the Si-H stretching mode at ∼2100 cm −1 .As the NH 3 /SiH 4 ratio is increased, the Si-H bond density drops and the peak shifts to higher wavenumbers, corresponding to a more nitrogen rich nitride films and larger concentration of K centers.

FIG. 5 .
FIG. 5. (a) Optical image of a monolayer WSe 2 back-gated transistor after SiN x deposition.(b) Transfer characteristics of the device before and after SiN x deposition.(c) Photoluminescence and (d) Raman spectra (excitation wavelength of 532 nm) of the monolayer before and after SiN x deposition indicating no damage to the flake occurred during nitride deposition.route towards controlled doping of thin layers of TMDCs.In the future, the use of dielectrics with negative fixed charges can be explored as a complementary p-doping scheme.Deposition of SiN x was funded by the NSF NASCENT Center.Other material processing and characterization was funded by the Director, Office of Science, Office of Basic Energy Sciences, Material Sciences and Engineering Division, U.S. Department of Energy, under Contract No. DE-AC02-05CH11231.Device fabrication was funded by the Center for Low Energy Systems Technology (LEAST), one of six centers supported by the STARnet phase of the Focus Center Research Program (FCRP), a Semiconductor Research Corporation program sponsored by MARCO and DARPA.